Twenty-nine workers in New Zealand are missing after a powerful blast tore through a coal mine. Repeated attempts to contact the 16 employees and 13 contract miners at the Pike River mine have failed, and it is not known if they are still alive.

GREYMOUTH, New Zealand -- Police acknowledged for the first time Monday that some of the 29 miners trapped underground for three days by a gas explosion may not have survived, as toxic fumes continued to prevent rescuers from entering the tunnel.

Authorities expected to finish drilling a six-inch-wide shaft into the mine tunnel later Monday to get a better idea of the air quality inside the mine in an area where miners were believed trapped by the Friday's blast. Tests every 30 minutes on areas currently accessible continued to indicate the air was too toxic for rescuers to enter.

"We still remain optimistic, we're still keeping an open mind," police superintendent Gary Knowles told reporters. "But we are planning for all outcomes, and as part of this process we're planning for the possible loss of life as a result of what's occurred underground."

Two workers stumbled out of the Pike River mine within hours of the explosion on Friday, but there has been no contact at all with the missing 29. A buildup of methane gas that was somehow ignited inside the mine is suspected as the cause, though officials say that won't be confirmed for days at least.

Officials say they will not send rescue teams into the mine until they are confident there is no danger of another blast. Knowles said there is also an unknown heat source underground.

The families of the miners were growing more concerned as time passed, wondering if the best was being done for their sons.

"Everybody's frustrated, everybody's upset," said Laurie Drew, whose 21-year-old son, Zen, was among the missing. "I have my moments I can keep it together but deep down my heart's bleeding like everybody else's."

Once the narrow test shaft is finished, experts will immediately begin taking a series of air samples to establish the gas levels at a part of the mine where some of the missing workers were believed to have been at the time of the explosion. Officials also will feed a very high-resolution laser camera down the hole to give rescuers their first sight of conditions -- and potentially the men inside.

The army, meanwhile, was preparing a robot normally used for bomb disposal to enter the mine to check conditions and take video inside.

Police have said the miners, aged 17 to 62, are believed to be about 1.2 miles down the tunnel.

Each miner carried 30 minutes of oxygen, and more fresh air was stored in the mine, along with food and water, that could allow several days of survival, officials say.